Super absorbent polymer (SAP) is synthetic polymer material that can absorb moisture of 500 to 1000 times of self-weight, and is also named differently as super absorbency material (SAM), absorbent gel material (AGM), etc. according to developing companies. The superabsorbent polymer began to be commercialized as sanitary items, and currently, it is being widely used as hygienic goods such as a disposable diaper and so on, water-holding material for soil, water stop material for civil engineering and architecture, sheets for raising seedling, freshness preservatives in the field of food circulation, fomentation material, etc.
As a method for preparing the superabsorbent polymer, reverse phase suspension polymerization or water-soluble solution polymerization, etc. are known. The reverse phase suspension polymerization is disclosed, for example, in Japanese Laid-Open Patent Publication No. Sho 56-161408, Japanese Laid-Open Patent Publication No. Sho 57-158209, and Japanese Laid-Open Patent Publication No. Sho 57-198714, etc. As the water-soluble solution polymerization, thermal polymerization wherein hydrogel polymer is polymerized while being fractured and cooled in a kneader equipped with many axes, and photopolymerization wherein a water-soluble solution of high concentration is irradiated by UV, etc. on a belt, thus simultaneously conducting polymerization and drying, are known.
Meanwhile, hydrogel polymer obtained through the polymerization is generally passed through a drying process and ground, and then, passed through a step of selectively crosslinking the polymer surface so as to have desired absorption capacity and absorbency under load. In the surface crosslinking, it is required to control the penetration depth of the surface crosslinking agent, and for this, in general, the penetration depth of the surface crosslinking agent was controlled using alcohol and water. However, in case the penetration depth is controlled using alcohol, a large quantity of alcohol is consumed, which is unfavorable in terms of cost, and additional drying treatment is required.
Therefore, in order to overcome the problems, methods of minimizing the amount of alcohol used or methods without using alcohol have been suggested. However, if the amount of alcohol used is excessively reduced, water may be excessively absorbed in the polymer, thus rendering efficient crosslinking difficult due to the agglomeration of gel.